1. Field of the Invention
This invention relates to a self curing single package inorganic zinc rich coating composition comprised of a binder prepared from a blend of a non-catalyzed silicate ester and an alkyl titanate. More particularly, the binder of the invention is formulated from a neutral or alkaline alkyl silicate ester having a low level of hydrolysis blended with an alkyl titanate ester wherein about 10% to 60% by weight of titanate is present as TiO.sub.2 to the total weight of TiO.sub.2 plus SiO.sub.2 in the copolymer blend.
2. Description of the Prior Art
Inorganic zinc rich paints have a long history of use in the protection of ferrous metals from corrosion in saline and atmospheric conditions that are not otherwise strongly acidic or alkaline. Zinc rich paint coating compositions consist basically of finely divided zinc dust or powder in a porous silicate matrix allowing electrical contact between the zinc dust and ferrous substrate to provide cathodic protection.
Since the development and introduction of these coatings around the time of World War II, research has been conducted to simplify their handling and application. The first coatings consisted of zinc dust and aqueous sodium silicate, which were cured by either baking at high temperatures or the post application of an acid solution to neutralize the alkali. Self curing coatings were developed later utilizing aqueous potassium, lithium or quarternary ammonium silicates as binders. Coatings of this type became insoluble upon drying.
A second method of providing zinc rich inorganic paints and coating compositions utilized organic solvents, usually alcoholic solutions of a partially hydrolyzed ethyl silicate. Coating compositions of this type are cured by the reaction of the ethyl silicate binder with atmospheric moisture.
Ethyl silicate, Si(OC.sub.2 H.sub.5).sub.4, functions as a binder in these reactions because it reacts with atmospheric moisture by a two-step hydrolysis and condensation reaction which is summarized by the equation: EQU Si(OC.sub.2 H.sub.5).sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4C.sub.2 H.sub.5 OH
This reaction has been too slow to be of practical value for coating compositions, so in practice the ethyl silicate binder has been pre-reacted by adding an acid stabilizer and approximately 80 to 90 percent of the amount of water required for a complete reaction. The acid, typically a strong mineral acid such as hydrochloric or sulfuric, catalyzes the hydrolysis reaction and renders the system metastable, presumably by forcing the silanol hydrolysis products to exist in the protonated form, less reactive than the nucleophilic ionized form as set forth in the following equation: ##STR1##
As long as such an acid is present, the system will tolerate amounts of water even greater than stoichiometric, condensing slowly, but as soon as the acid is neutralized and the system becomes alkaline, gelling occurs rapidly, presumably because of nucleophilic attack by ionized silanol groups on silicon atoms of other silicate or silanol groups to form siloxane linkages in accordance with the equation: ##STR2##
The acid stabilized binders are then formulated with neutral fillers such as talc, clay, mica, iron oxide, etc., and additional solvent, and packaged separately from the zinc dust. The zinc dust is stirred in just prior to application, and reacts with the acid stabilizer, causing the mixture to gel in about 8 to 24 hours. When the liquid mixture is applied to a surface, the solvent evaporates, the acid stabilizer reacts with the zinc or evaporates, and moisture as atmospheric humidity causes the system to cure.
The prior art zinc rich coating techniques and compositions have consequently shared the disadvantage of being two package materials requiring measuring, packaging and mixing just prior to the application of the coating. In addition, once the separate packages of zinc and carrier compositions were mixed, the coating had to be used within a relatively short time, usually a few hours. Material unused after the curing period had to be discarded. Other disadvantages of the prior art coating compositions included the uneconomical packaging and processing required for a two component system, the time spent packaging and then mixing and the resulting problems involved with using quantities other than in the prescribed package units.
It has long been a goal of the paint and coating industry to provide a coating composition which is an inorganic zinc rich single package coating composition having an acceptable shelf life and which is self curing upon exposure to the atmosphere. The single package inorganic zinc rich paints and coating compositions of the invention address these goals by providing a single package coating composition having shelf life stability and reasonable cure rates by employing a binder consisting of a mixture of alkyl silicate and alkyl titanate.